Page:Popular Science Monthly Volume 6.djvu/28

vertebrate animals, to use the word ganglion, which means a knot of nerves. Really, it signifies a little brain, so that an insect or a mollusk may have several brains in different parts of its body. It should be remembered, also, that in anatomy the forward or anterior end of an oyster is the part containing the mouth, and that is up against the hinge, while the posterior part happens in this case to be near the opening, or, as the oystermen call it, the nib of the shell. At b, then, we see the large brain of the oyster called the posterior ganglion. We see, too, that it is surrounded by nerves running to other parts of the structure. There are two curved branches, marked c c, which connect this brain with two comb-like objects. These are the nerves of the branchiæ or gills. This brain, then, has direct control of the mechanism and functions of respiration. But it is noticeable that it is

Fig. 8.—The Nervous System of an Oyster.

also connected with the entire system of the two nerve-lines, d d, which suggest the spinal cord of the vertebrates. And this double nerve-line crosses the two ganglia or little brains, a a, which are connected by the transverse nerve-branch e; thus the mouth, whose place is shown by the half-moon, is encircled by a nerve-ring, and this regulates the functions of ingestion. In those mollusks which travel, as do mussels and scallops, there is a ganglion or locomotive centre. Bearing this in mind, and the fact that the oyster does not have this ganglion, because it does not need it, not being a traveler at all, let us give the gist of Dr. Todd's remarks on the nervous system of the mollusca in general: "It affords a beautiful example of the complete analysis of the more complicated nervous system of the vertebrata. Have we not here distinctly marked out the cerebrum (the centre of volition and sensation), the medulla oblongata (the respiratory centre), and the cerebellum (the locomotive centre), as they occur in the